The present invention relates to electronic amplifiers in general, and more particularly, to low power video amplifiers.
A variety of specialized video amplifiers have been utilized in the art to provide a switch control interface between the display controller, of a digitally operated unit, for example, and the cathode of a cathode ray tube. The high voltage requirement for typical cathode ray tube electron beam guns mandates a significant voltage controlling capability. Additionally, and essentially in contradiction to this voltage handling requirement, the video amplifier is required to have a significant switching speed capability, in response to ever-increasing requirements for resolution and graphic clarity on the display.
These two conflicting requirements are met in some applications by utilizing a power transistor in series with a series resistor having a typical value of 1 kOhm. A variable brightness control transistor may be, in one embodiment, located between the series resistor and control transistor to control the voltage level of the electron gun, which controls the CRT brightness. This configuration has its limitations in that the series resistance must be of a relatively low value in order to obtain sufficient response at the cathode. Also, the same low value resistance results in a significantly higher power dissipation during ON conditions than is desirable, and as a direct result of the significant power consumption utilizing this low series resistance, heat buildup problems, bulk required by cooling fins, and weight problems, must also be addressed.
A second problem with this prior art series control technique is the inability to effectively offset the inherent capacitance in the wiring harness of the CRT and the interelectrode capacitance. As the ON/OFF speeds for the video amplifier increase, these stray capacitances become significant and therefore pose a practical limit for the maximum switching speed attainable with the series video amplifier control technique.